Jeff McLane and Zach Berman

Jason Peters was absent as the Eagles opened their third week of organized team activities on Tuesday.

Coach Chip Kelly said that he knew why Peters wasn't at practice and that it wasn't injury related.

"We know where he is, and we know what's going on," said Kelly, who added that he expected Peters back "shortly."

OTAs are voluntary, but Kelly has mostly had full attendance during the first two weeks of workouts. Peters, along with several other players, the Eagles said, missed last Monday's practice because of travel delays. Kelly, though, said that Peters missed all of last week's practice.

Peters is returning from a ruptured Achilles tendon that caused him to miss all of last season. He has participated in all facets of practicing this spring and Eagles offensive line coach Jeff Stoutland told reporters last week that it didn't look like Peters had even missed a beat.

Peters, 31, was considered by many to be the best left tackle in the game in 2011. He was voted into his fifth straight Pro Bowl that season. The Eagles line was best by injury last season. Center Jason Kelce (knee) and tackle Todd Herremans (foot) suffered season-ending injuries.

Evan Mathis had what the Eagles called minor ankle surgery earlier this month. He has missed practice since then, but was out working with the rest of his position group at the start of Tuesday's practice. He isn't expected to be 100 percent until training camp.

With Peters absent, Dennis Kelly played left tackle with the first team and rookie Lane Johnson moved up to take reps at right tackle. Todd Herremans and Allen Barbre were the first team right and left guards and Dallas Reynolds was at center.

There are three more days of OTAs this week and a three-day mandatory minicamp next week.